Knowledge of the in-situ stress field and pore pressure in sedimentary basins is fundamental to the analysis and prediction of geomechanical problems encountered in the petroleum industry. Examples of geomechanical problems include wellbore stability and fracturing of the formation during drilling that may lead to financial loss due to losses, kicks, stuck pipe, extra casing strings and sidetracks. Geomechanical problems may also include problems due to reservoir stress changes occurring during production such as reservoir compaction, surface subsidence, formation fracturing, casing deformation and failure, sanding, reactivation of faults and bedding parallel slip.
In general, the in-situ stress field may be represented as a second-rank tensor with three principal stresses. In the simplest case in which one of these is vertical, the three principal stresses are the vertical stress denoted by SV, the minimum horizontal stress denoted by Sh, and the maximum horizontal stress denoted by SH. The vertical stress may be estimated from an integral of the density log, while the minimum horizontal stress may be estimated using a poroelastic equation. The maximum horizontal stress is more difficult to evaluate. Accordingly, simple approximations are typically used to determine/estimate the value of the maximum horizontal stress.
The following are examples of the simple approximations used to determine/estimate the maximum horizontal stress: (i) approximating the maximum horizontal stress by setting it equal to the minimum horizontal stress; (ii) approximating the maximum horizontal stress by setting it equal to the average of the vertical stress and minimum horizontal stress; and (iii) approximating the maximum horizontal stress by setting it equal to the minimum horizontal stress multiplied by a factor greater than one. By approximating the value of maximum horizontal stress, errors may occur in the accuracy of the resulting values calculated using the approximations.